The invention relates to DC brush motors and, more particularly, to the reduction of DC motor noise when a low frequency pulse width modulated (PWM) signal is used for speed control.
A low speed frequency PWM signal provides a convenient signal to control the speed of a DC brush motor. However, the low frequency PWM signal creates motor torque fluctuations at the PWM frequency and harmonics that are transmitted to the supporting structure and hence creates noise and vibration. In engine cooling applications, the tones created by the constant low frequency PWM speed control degrade the product sound quality and are hence considered objectionable.
Accordingly, there is a need to improve the sound quality of a DC brush motor when low frequency PWM speed control is employed.
An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a method of controlling speed of a DC brush motor using a low frequency pulse width modulated (PWM) signal. The method provides a limited-pool of frequencies, with each frequency of the limited-pool being less than 500 Hz. A period of each of the frequencies of the limited-pool is determined and a pseudo-random sequence of the periods is created. A period from the pseudo-random sequence of periods is selected and the selected period is used as the period of the PWM signal to control speed of the DC brush motor.
In accordance with another aspect of the invention a motor control system includes an engine cooling module including a DC motor. The system includes a switch constructed and arranged to be controlled by a pulse width modulated (PWM) signal. The switch is operatively associated with the DC motor to control current to the DC motor and thus control the speed of the DC motor. A controller is operatively associated with the switch. The controller is constructed and arranged to select periods, of a limited-pool of frequencies less than 500 Hz, from a pseudo-random sequence of periods and to send the selected periods to the switch, thereby controlling noise and vibration of the module.
Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification.